There is no sushi bar per se, but sushi is available as a buffet item in the Jade section of the Windjammer. When you enter the Windjammer the first two serving lines you pass are "Jade" which features Asian/Indian cuisine. At lunch and during the evening buffet dinner Jade will have sushi dishes among other things. It is quite good actually.
But note that it is part of the larger Windjammer buffet, so you can mix and match foods and sit anywhere. Don't be surprised to see someone with a bowl of fried rice, California Rolls, chicken satay, and a cheeseburger and fries, all on the same tray.